Analyte sensors comprising leveling agents

ABSTRACT

Generally, embodiments of the invention relate to analyte determining methods and devices (e.g., electrochemical analyte monitoring systems) that have improved uniformity of distribution of one or more components of the sensor by inclusion of a low surface tension polymer leveling agent, where the components are disposed proximate to a working electrode of in vivo and/or in vitro analyte sensors, e.g., continuous and/or automatic in vivo monitoring using analyte sensors and/or test strips. Also provided are systems and methods of using the, for example electrochemical, analyte sensors in analyte monitoring.

BACKGROUND

In many instances it is desirable or necessary to regularly monitor the concentration of particular constituents in a fluid. A number of systems are available that analyze the constituents of bodily fluids such as blood, urine and saliva. Examples of such systems conveniently monitor the level of particular medically significant fluid constituents, such as, for example, cholesterol, ketones, vitamins, proteins, and various metabolites or blood sugars, such as glucose. Diagnosis and management of patients suffering from diabetes mellitus, a disorder of the pancreas where insufficient production of insulin prevents normal regulation of blood sugar levels, requires carefully monitoring of blood glucose levels on a daily basis. A number of systems that allow individuals to easily monitor their blood glucose are currently available. Such systems include electrochemical biosensors, including those that comprise a glucose sensor that is adapted for insertion into a subcutaneous site within the body for the continuous monitoring of glucose levels in bodily fluid of the subcutaneous site (see for example, U.S. Pat. No. 6,175,752 to Say et al).

A person may obtain the blood sample by withdrawing blood from a blood source in his or her body, such as a vein, using a needle and syringe, for example, or by lancing a portion of his or her skin, using a lancing device, for example, to make blood available external to the skin, to obtain the necessary sample volume for in vitro testing. The person may then apply the fresh blood sample to a test strip, whereupon suitable detection methods, such as calorimetric, electrochemical, or photometric detection methods, for example, may be used to determine the person's actual blood glucose level. The foregoing procedure provides a blood glucose concentration for a particular or discrete point in time, and thus, must be repeated periodically, in order to monitor blood glucose over a longer period.

In addition to the discrete or periodic, in vitro, blood glucose-monitoring systems described above, at least partially implantable, or in vivo, blood glucose-monitoring systems, which are constructed to provide continuous in vivo measurement of an individual's blood glucose concentration, have been described and developed.

Such analyte monitoring devices are constructed to provide for continuous or automatic monitoring of analytes, such as glucose, in the blood stream or interstitial fluid. Such devices include electrochemical sensors, at least a portion of which are operably positioned in a blood vessel or in the subcutaneous tissue of a patient.

While continuous glucose monitoring is desirable, there are several challenges associated with optimizing manufacture protocols to improve yield and uniformity of the sensing layer and/or other layer (e.g., an overlaying membrane) of the biosensors constructed for in vivo use. Accordingly, further development of manufacturing techniques and methods, as well as analyte-monitoring devices, systems, or kits employing the same, is desirable.

SUMMARY OF THE INVENTION

Generally, embodiments of the invention relate to analyte determining methods and devices (e.g., electrochemical analyte monitoring systems) that have improved uniformity of distribution of one or more components of the sensor by inclusion of a low surface tension polymer leveling agent, where the components are disposed proximate to a working electrode of in vivo and/or in vitro analyte sensors, e.g., continuous and/or automatic in vivo monitoring using analyte sensors and/or test strips. Also provided are systems and methods of using the, for example electrochemical, analyte sensors in analyte monitoring.

These and other objects, advantages, and features of the invention will become apparent to those persons skilled in the art upon reading the details of the invention as more fully described below.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of various aspects, features and embodiments of the invention are provided herein with reference to the accompanying drawings, which are briefly described below. The drawings are illustrative and are not necessarily drawn to scale. The drawings illustrate various aspects or features of the invention and may illustrate one or more embodiment(s) or example(s) of the invention in whole or in part. A reference numeral, letter, and/or symbol that is used in one drawing to refer to a particular element or feature maybe used in another drawing to refer to a like element or feature.

FIG. 1 shows a block diagram of an embodiment of a data monitoring and management system according to embodiments of the invention.

FIG. 2 shows a block diagram of an embodiment of the transmitter unit of the data monitoring and management system of FIG. 1.

FIG. 3 shows a block diagram of an embodiment of the receiver/monitor unit of the data monitoring and management system of FIG. 1.

FIG. 4 shows a schematic diagram of an embodiment of an analyte sensor according to the embodiments of the invention.

FIGS. 5A-5B show a perspective view and a cross sectional view, respectively of another embodiment an analyte sensor.

FIG. 6A is a microphotograph of six deposition spots of a sensing layer formulation on a gold surface. The thick, dark outer ring encircling each spot represents the formation of the “coffee ring” effect referred to herein. FIG. 6B is a microphotograph of six deposition spots of a sensing layer formulation on gold surface, wherein the sensing layer formulation comprises 2% of the low surface tension polymer leveling agent referred to as Dow Corning® 57 (DC 57). The microphotograph illustrates the extent of homogeneity, and uniformity of solution distribution after curing that results when a low surface tension polymer leveling agent is added to the sensing layer formulation.

FIG. 7 is a microphotograph of three deposition spots of a sensing layer formulation on a gold surface, wherein the sensing layer formulation comprises 2% of the low surface tension polymer leveling agent referred to as Dow Corning® 28 (DC 28). Again, the microphotograph illustrates the extent of homogeneity, and uniformity of solution distribution that results when a low surface tension polymer leveling agent is added to the sensing layer formulation.

FIG. 8 is a microphotograph of a single deposition spot of a sensing layer formulation on a gold surface, wherein the sensing layer formulation comprises 2% of the low surface tension polymer leveling agent referred to as Surfynol® 2502. Again, the microphotograph illustrates the extent of homogeneity, and uniformity of solution distribution that results when a low surface tension polymer leveling agent is added to the sensing layer formulation.

FIG. 9A, when juxtaposed against FIG. 9B, provides a comparison between a control sensor in which the membrane formulation was not supplemented with a low surface tension polymer leveling agent 10 days post cure at 56° C. (FIG. 6A), versus a sensor with a membrane coating that was treated with the low surface tension polymer dimethyl, methyl (polyethylene oxide acetate-capped) siloxane (“DC 57”) (FIG. 9B). After 16 days at 56° C., the leveling agent-treated sensor membrane appears smooth, homogenous and sturdy. FIG. 9B illustrates elimination of wrinkling and a leveling of the “orange peel” effect.

FIG. 10 shows the results of sensitivity comparison (nA/mm²) at varying concentrations of glucose (mM) between control sensors (diamonds), sensors with the sensing layers supplemented with 1% of the low surface tension polymer DC 57 (squares) and sensors with the sensing layers supplemented with 3% of the low surface tension polymer DC 57 (triangles).

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Before the embodiments of the invention are described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the embodiments of the invention will be limited only by the appended claims.

Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.

In the description of the invention herein, it will be understood that a word appearing in the singular encompasses its plural counterpart, and a word appearing in the plural encompasses its singular counterpart, unless implicitly or explicitly understood or stated otherwise. Merely by way of example, reference to “an” or “the” “analyte” encompasses a single analyte, as well as a combination and/or mixture of two or more different analytes, reference to “a” or “the” “concentration value” encompasses a single concentration value, as well as two or more concentration values, and the like, unless implicitly or explicitly understood or stated otherwise. Further, it will be understood that for any given component described herein, any of the possible candidates or alternatives listed for that component, may generally be used individually or in combination with one another, unless implicitly or explicitly understood or stated otherwise. Additionally, it will be understood that any list of such candidates or alternatives, is merely illustrative, not limiting, unless implicitly or explicitly understood or stated otherwise.

Various terms are described below to facilitate an understanding of the invention. It will be understood that a corresponding description of these various terms applies to corresponding linguistic or grammatical variations or forms of these various terms. It will also be understood that the invention is not limited to the terminology used herein, or the descriptions thereof, for the description of particular embodiments. Merely by way of example, the invention is not limited to particular analytes, bodily or tissue fluids, blood or capillary blood, or sensor constructs or usages, unless implicitly or explicitly understood or stated otherwise, as such may vary.

The publications discussed herein are provided solely for their disclosure prior to the filing date of the application. Nothing herein is to be construed as an admission that the embodiments of the invention are not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed.

Overview

Embodiments of the invention relate to methods and devices for improving the uniformity of distribution of one or more components of a sensor by inclusion of a low surface tension polymer leveling agent, where the components are disposed proximate to a working electrode of the sensor, such as in vivo and/or in vitro analyte sensors, including, such as, continuous and/or automatic in vivo analyte sensors. For example, embodiments of the invention provide for inclusion of a low surface tension polymer leveling agent in a solution, such as a sensing layer formulation and/or other layer (e.g., a membrane) formulation, resulting in reduction of the contact angle of the solution, thereby lowering the solution surface tension. Also provided are systems and methods of using the analyte sensors in analyte monitoring.

In general, embodiments of the invention are based on the discovery that the addition of a low surface tension polymer to solution formulations used in the manufacture of in vivo and/or in vitro biosensors greatly improves uniformity and/or distribution of one or more components of the sensor (e.g., an enzyme-containing sensing layer and/or one or more membrane layer(s) of such devices). Low surface tension polymers are additives that reduce the contact angle of the solution, thereby lowering the solution surface tension. The contact angle is the angle at which a liquid or vapor interface meets a solid surface. The contact angle is specific for any given system. On many highly hydrophilic surfaces, water droplets will exhibit contact angles of 0° to 30°. If the solid surface is hydrophobic, the contact angle will be larger than 90°. On highly hydrophobic surfaces, the surfaces have water contact angles as high as 150° or even nearly 180°. On these surfaces, water droplets simply rest on the surface, without actually wetting to any significant extent.

In general, the low surface tension polymers reduce the contact angle of the solution by migrating to the top of the solution and leveling the surface. The result is a reduction and in some cases complete elimination of the “coffee ring” effect (see homogenous and uniform distribution of each sample in FIG. 6B, FIG. 7 and FIG. 8). This results in a more uniform distribution of the constituents of a solution deposited on a substrate upon drying and curing, as well as a smoother surface of the solution upon drying and curing as compared to a solution lacking the low surface tension polymer. This, in turn, improves the coefficient of variation and the overall manufacturing process of the sensor and overall system.

Suitable low surface tension polymers for use with the described embodiments include, but are not limited to, polymeric compounds that reduce the overall surface tension of the solution to which they are added, thereby reducing the contact angle of the system between the substrate and the coating solution by about 3% to about 75%, including about 10% to about 50%, about 20% to about 40%, such as about 25% to about 35%, and the like.

Examples of low surface tension polymer leveling agents suitable for use with the subject methods, compositions and kits include, but are not limited to, polyacrylates and polysiloxanes with modified chain ends, including polyether-modified polydimethylsiloxanes. Examples for illustration purposes may include surfactants, surface-tension modifiers and/or surface-tension reducers such as Polydimethylsiloxane (H₃C)₃Si[Si(CH₃)₂O]_(n)Si(CH₃)₃; Polyether modified Polydimethylsiloxane [CH₂O—]_(n)[Si(CH₃)₂O]_(n)—[CH₂O—]_(n); Poly((ethylene oxide)(polyethylene oxide))butyl terminated PDMS CH₃—{[CH₂CH₂O]_(n)—[CH₂O—]_(n)}_(n)[Si(CH₃)₂O]_(n)—{[CH₂O—]_(n)[CH₂CH₂O]_(n)}_(n)—CH₃; and Dimethyl, methyl (polyethylene oxide acetate-capped) siloxane (DC 57), wherein n is an integer in a range of from about 1 to 500. An additional example of a low surface tension polymer leveling agents includes Surfynol 2502 (Air Products and Chemicals, Inc., Allentown, Pa.; CAS Number: 182211-02-5), which may include alkoxylated acetylinic diol ether or ethylenic diol modified with PEO (e.g., 5 moles) and PPO (e.g., 2 moles), oxirane, or 1,4-dimethyl-1,4-bis(2-methylpropyl)-2-butyne-1,4-diyl ether.

The low surface tension polymer leveling agent may be included in any component of a sensor that can benefit from improvement of adhesion and/or uniformity of distribution. Exemplary components include, but are not limited to, formulations that provide reagents such as enzyme or the like, such as a sensing layer having an analyte responsive enzyme, and one or more membrane layers that are used, inter alia, to promote one or more functionality to the sensor, such as minimize interfering substance and/or promote bio-compatibility and/or regulate-analyte (e.g., glucose) flux to in vivo biosensors for continuous glucose monitoring. Such components may be sensitive to the formation of crinkles and creases upon curing, giving an “orange peel” effect, such that the surface of the layer may resemble an orange peel. In addition, the component formulation of a sensor when contacted to the sensor (e.g., by dip coating, spray coating, or the like) and cured may form a brittle shell. This phenomenon may give the device a brittleness that may cause it to crack, break down and/or peel off of the substrate. These characteristics may cause the membrane to slough, chip and peel off carbon and other substrates. In some instances, this chipping can result in the undesirable deposition of residual pieces of the membrane in vivo. In addition, the components of a solution are also sensitive to migrating and settling along the outer perimeter of the deposition, referred to as formation of a “coffee ring” effect on the substrate.

Additional exemplary components of a sensor that may be suitably formulated with a low surface tension polymer leveling agent are described in U.S. Pat. Nos. 5,262,035, 5,262,305, 6,134,461, 6,143,164, 6,175,752, 6,338,790, 6,579,690, 6,654,625, 6,736,957, 6,746,582, 6,932,894, 6,605,200, 6,605,201, 7,090,756, 6,746,582 as well as those described in U.S. patent application Ser. Nos. 11/701,138, 11/948,915, all of which are incorporated herein by reference in their entirety.

In some embodiments, the low surface tension polymer leveling agent is formulated with a sensing layer that is disposed proximate to the working electrode. Generally, an embodiment of a sensing layer may be described as the area shown schematically in FIG. 5B as 508. The sensing layer may be described as the active chemical area of the biosensor. The sensing layer formulation, which can include a glucose-transducing agent, may include, for example, among other constituents, a redox mediator, such as, for example, a hydrogen peroxide or a transition metal complex, such as a ruthenium-containing complex or an osmium-containing complex, and an analyte response enzyme, such as, for example, a glucose responsive enzyme (e.g., glucose oxidase, glucose dehydrogenase, etc.) or lactate responsive enzyme (e.g., lactate oxidase). The sensing layer may also include other optional components, such as, for example, a polymer and a bi-functional, short-chain, epoxide cross-linker, such as PEG.

Any suitable proportion of low surface tension polymer may be used with a sensing layer, where the specifics will depend on, e.g., the particular sensing layer, etc. In certain embodiments, the low surface tension polymer leveling agents may comprise from about 0.5% to about 60% by weight of the total biosensor sensing layer formulation. For example, such low surface tension polymer leveling agents may comprise from about 1% to about 30% by weight of the total biosensor sensing layer formulation, including, for example, about 5% to about 25%, about 8% to about 22%, about 10% to about 20%, about 12% to about 18%, about 14% to about 16%, and the like.

In other embodiments, in addition to or instead of being employed with a sensing layer, a low surface tension polymer leveling agent may be formulated with one or more other layers of a sensor, e.g., a membrane that is disposed over at least a portion of the sensing layer of the working electrode. Any suitable proportion of low surface tension polymer may be used with such one or more layers, where the specifics will depend on, e.g., the particular layer, etc. In certain embodiments, the low surface tension polymer leveling agent may comprise from about 0.001% to about 10% by weight of the total biosensor membrane formulation. For example, such low surface tension polymer leveling agents may comprise from about 0.001% to about 1% by weight of the total biosensor membrane formulation, including, for example, about 0.01% to about 0.9%, about 0.05% to about 0.5%, about 0.08% to about 0.2%, about 0.09% to about 0.1%, and the like.

In an electrochemical embodiment, the sensor is placed, transcutaneously, for example, into a subcutaneous site such that subcutaneous fluid of the site comes into contact with the sensor. In other in vivo embodiments, placement of at least a portion of the sensor may be in a blood vessel. The sensor operates to electrolyze an analyte of interest in the subcutaneous fluid such that a current is generated between the working electrode and the counter electrode. A value for the current associated with the working electrode is determined. If multiple working electrodes are used, current values from each of the working electrodes may be determined. A microprocessor may be used to collect these periodically determined current values or to further process these values.

If an analyte concentration is successfully determined, it may be displayed, stored, and/or otherwise processed to provide useful information. By way of example, raw signal or analyte concentrations may be used as a basis for determining a rate of change in analyte concentration, which should not change at a rate greater than a predetermined threshold amount. If the rate of change of analyte concentration exceeds the predefined threshold, an indication maybe displayed or otherwise transmitted to indicate this fact.

As demonstrated herein, the methods of the invention are particularly useful in connection with a device that is used to measure or monitor a glucose analyte, such as any such device described herein. These methods may also be used in connection with a device that is used to measure or monitor another analyte, including oxygen, carbon dioxide, proteins, drugs, or another moiety of interest, for example, or any combination thereof, found in bodily fluid, including subcutaneous fluid, dermal fluid (sweat, tears, and the like), interstitial fluid, or other bodily fluid of interest, for example, or any combination thereof. In general, the device is in good contact, such as thorough and substantially continuous contact, with the bodily fluid.

According to an embodiment of the invention, the measurement sensor is one suited for electrochemical measurement of analyte concentration, such as, for example, glucose concentration, in a bodily fluid. In this embodiment, the measurement sensor comprises at least a working electrode and a counter electrode. Another embodiment may further comprise a reference electrode. The working electrode is typically associated with a glucose-responsive enzyme. A mediator may also be included. In certain embodiments, hydrogen peroxide, which may be characterized as a mediator, is produced by a reaction of the sensor and may be used to infer the concentration of glucose. In some embodiments, a mediator is added to the sensor by a manufacturer, i.e., is included with the sensor even prior to use. Generally, a redox mediator is relative to the working electrode and is capable of transferring electrons between a compound and a working electrode, either directly or indirectly. Merely by way of example, the redox mediator may be, and is, for example, immobilized on the working electrode, e.g., entrapped on a surface or chemically bound to a surface.

Electrochemical Sensors

Embodiments of the invention relate to methods and devices for detecting at least one analyte, including glucose, in body fluid. Embodiments relate to the continuous and/or automatic in vivo monitoring of the level of one or more analytes using a continuous analyte monitoring system that includes an analyte sensor at least a portion of which is to be positioned beneath a skin surface of a user for a period of time and/or the discrete monitoring of one or more analytes using an in vitro blood glucose (“BG”) meter and an analyte test strip. Embodiments include combined or combinable devices, systems and methods and/or transferring data between an in vivo continuous system and an in vivo system. In some embodiments, the systems, or at least a portion of the systems, are integrated into a single unit.

A sensor that includes a low surface tension polymer may be an in vivo sensor or an in vitro sensor (i.e., a discrete monitoring test strip). Such a sensor can be formed on a substrate, e.g., a substantially planar substrate. In certain embodiments, such a sensor is a wire, e.g., a working electrode wire inner portion with one or more other electrodes associated (e.g., on, including wrapped around) therewith. The sensor may also include at least one counter electrode (or counter/reference electrode) and/or at least one reference electrode or at least one reference/counter electrode.

Accordingly, embodiments include analyte monitoring devices and systems that include an analyte sensor at least a portion of which is positionable beneath the skin of the user for the in vivo detection, of an analyte, including glucose, lactate, and the like, in a body fluid. Embodiments include wholly implantable analyte sensors and analyte sensors in which only a portion of the sensor is positioned under the skin and a portion of the sensor resides above the skin, e.g., for contact to a sensor control unit (which may include a transmitter), a receiver/display unit, transceiver, processor, etc. The sensor may be, for example, subcutaneously positionable in a patient for the continuous or periodic monitoring of a level of an analyte in a patient's interstitial fluid. For the purposes of this description, continuous monitoring and periodic monitoring will be used interchangeably, unless noted otherwise. The sensor response may be correlated and/or converted to analyte levels in blood or other fluids. In certain embodiments, an analyte sensor may be positioned in contact with interstitial fluid to detect the level of glucose, which detected glucose may be used to infer the glucose level in the patient's bloodstream. Analyte sensors may be insertable into a vein, artery, or other portion of the body containing fluid. Embodiments of the analyte sensors of the subject invention having a low surface tension polymer may be configured for monitoring the level of the analyte over a time period which may range from seconds, minutes, hours, days, weeks, to months, or longer.

Of interest are analyte sensors, such as glucose sensors, having a low surface tension polymer, that are capable of in vivo detection of an analyte for about one hour or more, e.g., about a few hours or more, e.g., about a few days or more, e.g., about three or more days, e.g., about five days or more, e.g., about seven days or more, e.g., about several weeks or at least one month or more. Future analyte levels may be predicted based on information obtained, e.g., the current analyte level at time to, the rate of change of the analyte, etc. Predictive alarms may notify the user of a predicted analyte levels that may be of concern in advance of the user's analyte level reaching the future level. This provides the user an opportunity to take corrective action.

FIG. 1 shows a data monitoring and management system such as, for example, an analyte (e.g., glucose) monitoring system 100 in accordance with certain embodiments. Embodiments of the subject invention are further described primarily with respect to glucose monitoring devices and systems, and methods of glucose detection, for convenience only and such description is in no way intended to limit the scope of the invention. It is to be understood that the analyte monitoring system may be configured to monitor a variety of analytes at the same time or at different times.

Analytes that may be monitored include, but are not limited to, acetyl choline, amylase, bilirubin, cholesterol, chorionic gonadotropin, creatine kinase (e.g., CK-MB), creatine, creatinine, DNA, fructosamine, glucose, glutamine, growth hormones, hormones, ketone bodies, lactate, peroxide, prostate-specific antigen, prothrombin, RNA, thyroid stimulating hormone, and troponin. The concentration of drugs, such as, for example, antibiotics (e.g., gentamicin, vancomycin, and the like), digitoxin, digoxin, drugs of abuse, theophylline, and warfarin, may also be monitored. In those embodiments that monitor more than one analyte, the analytes may be monitored at the same or different times.

The analyte monitoring system 100 includes a sensor 101, a data processing unit 102 connectable to the sensor 101, and a primary receiver unit 104 which is configured to communicate with the data processing unit 102 via a communication link 103. In certain embodiments, the primary receiver unit 104 may be further configured to transmit data to a data processing terminal 105 to evaluate or otherwise process or format data received by the primary receiver unit 104. The data processing terminal 105 may be configured to receive data directly from the data processing unit 102 via a communication link which may optionally be configured for bi-directional communication. Further, the data processing unit 102 may include a transmitter or a transceiver to transmit and/or receive data to and/or from the primary receiver unit 104 and/or the data processing terminal 105 and/or optionally the secondary receiver unit 106.

Also shown in FIG. 1 is an optional secondary receiver unit 106 which is operatively coupled to the communication link and configured to receive data transmitted from the data processing unit 102. The secondary receiver unit 106 may be configured to communicate with the primary receiver unit 104, as well as the data processing terminal 105. The secondary receiver unit 106 may be configured for bi-directional wireless communication with each of the primary receiver unit 104 and the data processing terminal 105. As discussed in further detail below, in certain embodiments the secondary receiver unit 106 may be a de-featured receiver as compared to the primary receiver, i.e., the secondary receiver may include a limited or minimal number of functions and features as compared with the primary receiver unit 104. As such, the secondary receiver unit 106 may include a smaller (in one or more, including all, dimensions), compact housing or embodied in a device including a wrist watch, arm band, PDA, etc., for example. Alternatively, the secondary receiver unit 106 may be configured with the same or substantially similar functions and features as the primary receiver unit 104. The secondary receiver unit 106 may include a docking portion to be mated with a docking cradle unit for placement by, e.g., the bedside for night time monitoring, and/or a bi-directional communication device. A docking cradle may recharge a powers supply.

Only one sensor 101, data processing unit 102 and data processing terminal 105 are shown in the embodiment of the analyte monitoring system 100 illustrated in FIG. 1. However, it will be appreciated by one of ordinary skill in the art that the analyte monitoring system 100 may include more than one sensor 101 and/or more than one data processing unit 102, and/or more than one data processing terminal 105. Multiple sensors may be positioned in a patient for analyte monitoring at the same or different times. In certain embodiments, analyte information obtained by a first positioned sensor may be employed as a comparison to analyte information obtained by a second sensor. This may be useful to confirm or validate analyte information obtained from one or both of the sensors. Such redundancy may be useful if analyte information is contemplated in critical therapy-related decisions. In certain embodiments, a first sensor may be used to calibrate a second sensor.

The analyte monitoring system 100 may be a continuous monitoring system, or semi-continuous, or a discrete monitoring system. In a multi-component environment, each component may be configured to be uniquely identified by one or more of the other components in the system so that communication conflict may be readily resolved between the various components within the analyte monitoring system 100. For example, unique IDs, communication channels, and the like, may be used.

In certain embodiments, the sensor 101 is physically positioned in or on the body of a user whose analyte level is being monitored. The sensor 101 may be configured to at least periodically sample the analyte level of the user and convert the sampled analyte level into a corresponding signal for transmission by the data processing unit 102. The data processing unit 102 is coupleable to the sensor 101 so that both devices are positioned in or on the user's body, with at least a portion of the analyte sensor 101 positioned transcutaneously. The data processing unit may include a fixation element such as adhesive or the like to secure it to the user's body. A mount (not shown) attachable to the user and mateable with the unit 102 may be used. For example, a mount may include an adhesive surface. The data processing unit 102 performs data processing functions, where such functions may include but are not limited to, filtering and encoding of data signals, each of which corresponds to a sampled analyte level of the user, for transmission to the primary receiver unit 104 via the communication link 103. In one embodiment, the sensor 101 or the data processing unit 102 or a combined sensor/data processing unit may be wholly implantable under the skin layer of the user.

In certain embodiments, the primary receiver unit 104 may include an analog interface section including and RF receiver and an antenna that is configured to communicate with the data processing unit 102 via the communication link 103, and a data processing section for processing the received data from the data processing unit 102 including data decoding, error detection and correction, data clock generation, data bit recovery, etc., or any combination thereof.

In operation, the primary receiver unit 104 in certain embodiments is configured to synchronize with the data processing unit 102 to uniquely identify the data processing unit 102, based on, for example, an identification information of the data processing unit 102, and thereafter, to periodically receive signals transmitted from the data processing unit 102 associated with the monitored analyte levels detected by the sensor 101.

Referring again to FIG. 1, the data processing terminal 105 may include a personal computer, a portable computer including a laptop or a handheld device (e.g., personal digital assistants (PDAs), telephone including a cellular phone (e.g., a multimedia and Internet-enabled mobile phone including an iPhone™, iPOD™ or similar phone), mp3 player, pager, and the like), drug delivery device, each of which may be configured for data communication with the receiver via a wired or a wireless connection. Additionally, the data processing terminal 105 may further be connected to a data network (not shown) for storing, retrieving, updating, and/or analyzing data corresponding to the detected analyte level of the user.

The data processing terminal 105 may include an infusion device such as an insulin infusion pump or the like, which may be configured to administer insulin to patients, and which may be configured to communicate with the primary receiver unit 104 for receiving, among others, the measured analyte level. Alternatively, the primary receiver unit 104 may be configured to integrate an infusion device therein so that the primary receiver unit 104 is configured to administer insulin (or other appropriate drug) therapy to patients, for example, for administering and modifying basal profiles, as well as for determining appropriate boluses for administration based on, among others, the detected analyte levels received from the data processing unit 102. An infusion device may be an external device or an internal device (wholly implantable in a user).

In certain embodiments, the data processing terminal 105, which may include an insulin pump, may be configured to receive the analyte signals from the data processing unit 102, and thus, incorporate the functions of the primary receiver unit 104 including data processing for managing the patient's insulin therapy and analyte monitoring. In certain embodiments, the communication link 103 as well as one or more of the other communication interfaces shown in FIG. 1, may use one or more of: an RF communication protocol, an infrared communication protocol, a Bluetooth enabled communication protocol, an 802.11x wireless communication protocol, or an equivalent wireless communication protocol which would allow secure, wireless communication of several units (for example, per HIPPA requirements), while avoiding potential data collision and interference.

FIG. 2 shows a block diagram of an embodiment of a data processing unit of the data monitoring and detection system shown in FIG. 1. User input and/or interface components may be included or a data processing unit may be free of user input and/or interface components. In certain embodiments, one or more application-specific integrated circuits (ASIC) may be used to implement one or more functions or routins associated with the operations of the data processing unit (and/or receiver unit) using for example one or more state machines and buffers.

As can be seen in the embodiment of FIG. 2, the sensor unit 101 (FIG. 1) includes four contacts, three of which are electrodes—work electrode (W) 210, reference electrode (R) 212, and counter electrode (C) 213, each operatively coupled to the analog interface 201 of the data processing unit 102. This embodiment also shows optional guard contact (G) 211. Fewer or greater electrodes may be employed. For example, the counter and reference electrode functions may be served by a single counter/reference electrode, there may be more than one working electrode and/or reference electrode and/or counter electrode, etc.

FIG. 3 is a block diagram of an embodiment of a receiver/monitor unit such as the primary receiver unit 104 of the data monitoring and management system shown in FIG. 1. The primary receiver unit 104 includes one or more of: a blood glucose test strip interface 301, an RF receiver 302, an input 303, a temperature detection section 304, and a clock 305, each of which is operatively coupled to a processing and storage section 307. The primary receiver unit 104 also includes a power supply 306 operatively coupled to a power conversion and monitoring section 308. Further, the power conversion and monitoring section 308 is also coupled to the receiver processor 307. Moreover, also shown are a receiver serial communication section 309, and an output 310, each operatively coupled to the processing and storage unit 307. The receiver may include user input and/or interface components or may be free of user input and/or interface components.

In certain embodiments, the test strip interface 301 includes a glucose level testing portion to receive a blood (or other body fluid sample) glucose test or information related thereto. For example, the interface may include a test strip port to receive a glucose test strip. The device may determine the glucose level of the test strip, and optionally display (or otherwise notice) the glucose level on the output 310 of the primary receiver unit 104. Any suitable test strip may be employed, e.g., test strips that only require a very small amount (e.g., one microliter or less, e.g., 0.5 microliter or less, e.g., 0.1 microliter or less), of applied sample to the strip in order to obtain accurate glucose information, e.g. FreeStyle® blood glucose test strips from Abbott Diabetes Care, Inc. Glucose information obtained by the in vitro glucose testing device may be used for a variety of purposes, computations, etc. For example, the information may be used to calibrate sensor 101, confirm results of the sensor 101 to increase the confidence thereof (e.g., in instances in which information obtained by sensor 101 is employed in therapy related decisions), etc.

In further embodiments, the data processing unit 102 and/or the primary receiver unit 104 and/or the secondary receiver unit 105, and/or the data processing terminal/infusion section 105 may be configured to receive the blood glucose value wirelessly over a communication link from, for example, a blood glucose meter. In further embodiments, a user manipulating or using the analyte monitoring system 100 (FIG. 1) may manually input the blood glucose value using, for example, a user interface (for example, a keyboard, keypad, voice commands, and the like) incorporated in the one or more of the data processing unit 102, the primary receiver unit 104, secondary receiver unit 105, or the data processing terminal/infusion section 105.

Additional detailed descriptions are provided in U.S. Pat. Nos. 5,262,035; 5,264,104; 5,262,305; 5,320,715; 5,593,852; 6,175,752; 6,650,471; 6,746, 582, and in application Ser. No. 10/745,878 filed Dec. 26, 2003 entitled “Continuous Glucose Monitoring System and Methods of Use”, each of which is incorporated herein by reference.

FIG. 4 schematically shows an embodiment of an analyte sensor in accordance with the embodiments of the invention. This sensor embodiment includes electrodes 401, 402 and 403 on a base 404. Electrodes (and/or other features) may be applied or otherwise processed using any suitable technology, e.g., chemical vapor deposition (CVD), physical vapor deposition, sputtering, reactive sputtering, printing, coating, ablating (e.g., laser ablation), painting, dip coating, etching, and the like. Materials include, but are not limited to, any one or more of aluminum, carbon (including graphite), cobalt, copper, gallium, gold, indium, iridium, iron, lead, magnesium, mercury (as an amalgam), nickel, niobium, osmium, palladium, platinum, rhenium, rhodium, selenium, silicon (e.g., doped polycrystalline silicon), silver, tantalum, tin, titanium, tungsten, uranium, vanadium, zinc, zirconium, mixtures thereof, and alloys, oxides, or metallic compounds of these elements.

The sensor may be wholly implantable in a user or may be configured so that only a portion is positioned within (internal) a user and another portion outside (external) a user. For example, the sensor 400 may include a portion positionable above a surface of the skin 410, and a portion positioned below the skin. In such embodiments, the external portion may include contacts (connected to respective electrodes of the second portion by traces) to connect to another device also external to the user such as a transmitter unit. While the embodiment of FIG. 4 shows three electrodes side-by-side on the same surface of base 404, other configurations are contemplated, e.g., fewer or greater electrodes, some or all electrodes on different surfaces of the base or present on another base, some or all electrodes stacked together, electrodes of differing materials and dimensions, etc.

FIG. 5A shows a perspective view of an embodiment of an electrochemical analyte sensor 500 having a first portion (which in this embodiment may be characterized as a major portion) positionable above a surface of the skin 510, and a second portion (which in this embodiment may be characterized as a minor portion) that includes an insertion tip 530 positionable below the skin, e.g., penetrating through the skin and into, e.g., the subcutaneous space 520, in contact with the user's biofluid such as interstitial fluid. Contact portions of a working electrode 501, a reference electrode 502, and a counter electrode 503 are positioned on the portion of the sensor 500 situated above the skin surface 510. Working electrode 501, a reference electrode 502, and a counter electrode 503 are shown at the second section and particularly at the insertion tip 530. Traces may be provided from the electrode at the tip to the contact, as shown in FIG. 5A. It is to be understood that greater or fewer electrodes may be provided on a sensor. For example, a sensor may include more than one working electrode and/or the counter and reference electrodes may be a single counter/reference electrode, etc.

FIG. 5B shows a cross sectional view of a portion of the sensor 500 of FIG. 5A. The electrodes 501, 502 and 503, of the sensor 500 as well as the substrate and the dielectric layers are provided in a layered configuration or construction. For example, as shown in FIG. 5B, in one aspect, the sensor 500 (such as the sensor unit 101 FIG. 1), includes a substrate layer 504, and a first conducting layer 501 such as carbon, gold, etc., disposed on at least a portion of the substrate layer 504, and which may provide the working electrode. Also shown disposed on at least a portion of the first conducting layer 501 is a sensing layer 508.

A first insulation layer such as a first dielectric layer 505 is disposed or layered on at least a portion of the first conducting layer 501, and further, a second conducting layer 509 may be disposed or stacked on top of at least a portion of the first insulation layer (or dielectric layer) 505. As shown in FIG. 5B, the second conducting layer 509 may provide the reference electrode 502, as described herein having an extended lifetime, which includes a layer of redox polymer as described herein.

A second insulation layer 506 such as a dielectric layer in one embodiment may be disposed or layered on at least a portion of the second conducting layer 509. Further, a third conducting layer 503 may provide the counter electrode 503. It may be disposed on at least a portion of the second insulation layer 506. Finally, a third insulation layer may be disposed or layered on at least a portion of the third conducting layer 503. In this manner, the sensor 500 may be layered such that at least a portion of each of the conducting layers is separated by a respective insulation layer (for example, a dielectric layer). The embodiment of FIGS. 5A and 5B show the layers having different lengths. Some or all of the layers may have the same or different lengths and/or widths.

In certain embodiments, some or all of the electrodes 501, 502, 503 may be provided on the same side of the substrate 504 in the layered construction as described above, or alternatively, may be provided in a co-planar manner such that two or more electrodes may be positioned on the same plane (e.g., side-by side (e.g., parallel) or angled relative to each other) on the substrate 504. For example, co-planar electrodes may include a suitable spacing there between and/or include dielectric material or insulation material disposed between the conducting layers/electrodes. Furthermore, in certain embodiments one or more of the electrodes 501, 502, 503 may be disposed on opposing sides of the substrate 504. In such embodiments, contact pads may be one the same or different sides of the substrate. For example, an electrode may be on a first side and its respective contact may be on a second side, e.g., a trace connecting the electrode and the contact may traverse through the substrate.

As noted above, analyte sensors may include an analyte-responsive enzyme to provide a sensing component or sensing layer. Some analytes, such as oxygen, can be directly electrooxidized or electroreduced on a sensor, and more specifically at least on a working electrode of a sensor. Other analytes, such as glucose and lactate, require the presence of at least one electron transfer agent and/or at least one catalyst to facilitate the electrooxidation or electroreduction of the analyte. Catalysts may also be used for those analytes, such as oxygen, that can be directly electrooxidized or electroreduced on the working electrode. For these analytes, each working electrode includes a sensing layer (see for example sensing layer 408 of FIG. 5B) proximate to or on a surface of a working electrode. In many embodiments, a sensing layer is formed near or on only a small portion of at least a working electrode.

The sensing layer includes one or more components constructed to facilitate the electrochemical oxidation or reduction of the analyte. The sensing layer may include, for example, a catalyst to catalyze a reaction of the analyte and produce a response at the working electrode, an electron transfer agent to transfer electrons between the analyte and the working electrode (or other component), or both.

A variety of different sensing layer configurations may be used. In certain embodiments, the sensing layer is deposited on the conductive material of a working electrode. The sensing layer may extend beyond the conductive material of the working electrode. In some cases, the sensing layer may also extend over other electrodes, e.g., over the counter electrode and/or reference electrode (or counter/reference is provided).

A sensing layer that is in direct contact with the working electrode may contain an electron transfer agent to transfer electrons directly or indirectly between the analyte and the working electrode, and/or a catalyst to facilitate a reaction of the analyte. For example, a glucose, lactate, or oxygen electrode may be formed having a sensing layer which contains a catalyst, including glucose oxidase, glucose dehydrogenase, lactate oxidase, or laccase, respectively, and an electron transfer agent that facilitates the electrooxidation of the glucose, lactate, or oxygen, respectively.

In other embodiments the sensing layer is not deposited directly on the working electrode. Instead, the sensing layer 64 may be spaced apart from the working electrode, and separated from the working electrode, e.g., by a separation layer. A separation layer may include one or more membranes or films or a physical distance. In addition to separating the working electrode from the sensing layer the separation layer may also act as a mass transport limiting layer and/or an interferent eliminating layer and/or a biocompatible layer.

In certain embodiments which include more than one working electrode, one or more of the working electrodes may not have a corresponding sensing layer, or may have a sensing layer which does not contain one or more components (e.g., an electron transfer agent and/or catalyst) needed to electrolyze the analyte. Thus, the signal at this working electrode may correspond to background signal which may be removed from the analyte signal obtained from one or more other working electrodes that are associated with fully-functional sensing layers by, for example, subtracting the signal.

In certain embodiments, the sensing layer includes one or more electron transfer agents. Electron transfer agents that may be employed are electroreducible and electrooxidizable ions or molecules having redox potentials that are a few hundred millivolts above or below the redox potential of the standard calomel electrode (SCE). The electron transfer agent may be organic, organometallic, or inorganic. Examples of organic redox species are quinones and species that in their oxidized state have quinoid structures, such as Nile blue and indophenol. Examples of organometallic redox species are metallocenes including ferrocene. Examples of inorganic redox species are hexacyanoferrate (III), ruthenium hexamine etc. Additional examples include those described in U.S. Pat. No. 6,736,957 and U.S. Patent Publication Nos. 2004/0079653 2006/0201805, the disclosures of which are incorporated herein by reference in their entirety.

In certain embodiments, electron transfer agents have structures or charges which prevent or substantially reduce the diffusional loss of the electron transfer agent during the period of time that the sample is being analyzed. For example, electron transfer agents include but are not limited to a redox species, e.g., bound to a polymer which can in turn be disposed on or near the working electrode. The bond between the redox species and the polymer may be covalent, coordinative, or ionic. Although any organic, organometallic or inorganic redox species may be bound to a polymer and used as an electron transfer agent, in certain embodiments the redox species is a transition metal compound or complex, e.g., osmium, ruthenium, iron, and cobalt compounds or complexes. It will be recognized that many redox species described for use with a polymeric component may also be used, without a polymeric component.

One type of polymeric electron transfer agent contains a redox species covalently bound in a polymeric composition. An example of this type of mediator is poly(vinylferrocene). Another type of electron transfer agent contains an ionically-bound redox species. This type of mediator may include a charged polymer coupled to an oppositely charged redox species. Examples of this type of mediator include a negatively charged polymer coupled to a positively charged redox species such as an osmium or ruthenium polypyridyl cation. Another example of an ionically-bound mediator is a positively charged polymer including quaternized poly(4-vinyl pyridine) or poly(1-vinyl imidazole) coupled to a negatively charged redox species such as ferricyanide or ferrocyanide. In other embodiments, electron transfer agents include a redox species coordinatively bound to a polymer. For example, the mediator may be formed by coordination of an osmium or cobalt 2,2′-bipyridyl complex to poly(1-vinyl imidazole) or poly(4-vinyl pyridine).

Suitable electron transfer agents are osmium transition metal complexes with one or more ligands, each ligand having a nitrogen-containing heterocycle such as 2,2′-bipyridine, 1,10-phenanthroline, 1-methyl, 2-pyridyl biimidazole, or derivatives thereof. The electron transfer agents may also have one or more ligands covalently bound in a polymer, each ligand having at least one nitrogen-containing heterocycle, such as pyridine, imidazole, or derivatives thereof. One example of an electron transfer agent includes (a) a polymer or copolymer having pyridine or imidazole functional groups and (b) osmium cations complexed with two ligands, each ligand containing 2,2′-bipyridine, 1,10-phenanthroline, or derivatives thereof, the two ligands not necessarily being the same. Some derivatives of 2,2′-bipyridine for complexation with the osmium cation include but are not limited to 4,4′-dimethyl-2,2′-bipyridine and mono-, di-, and polyalkoxy-2,2′-bipyridines, including 4,4′-dimethoxy-2,2′-bipyridine. Derivatives of 1,10-phenanthroline for complexation with the osmium cation include but are not limited to 4,7-dimethyl-1,10-phenanthroline and mono, di-, and polyalkoxy-1,10-phenanthrolines, such as 4,7-dimethoxy-1,10-phenanthroline. Polymers for complexation with the osmium cation include but are not limited to polymers and copolymers of poly(1-vinyl imidazole) (referred to as “PVI”) and poly(4-vinyl pyridine) (referred to as “PVP”). Suitable copolymer substituents of poly(1-vinyl imidazole) include acrylonitrile, acrylamide, and substituted or quaternized N-vinyl imidazole, e.g., electron transfer agents with osmium complexed to a polymer or copolymer of poly(1-vinyl imidazole).

Embodiments may employ electron transfer agents having a redox potential ranging from about −200 mV to about +200 mV versus the standard calomel electrode (SCE). The sensing layer may also include a catalyst which is capable of catalyzing a reaction of the analyte. The catalyst may also, in some embodiments, act as an electron transfer agent. One example of a suitable catalyst is an enzyme which catalyzes a reaction of the analyte. For example, a catalyst, including a glucose oxidase, glucose dehydrogenase (e.g., pyrroloquinoline quinone (PQQ), dependent glucose dehydrogenase, flavine adenine dinucleotide (FAD) dependent glucose dehydrogenase, or nicotinamide adenine dinucleotide (NAD) dependent glucose dehydrogenase), may be used when the analyte of interest is glucose. A lactate oxidase or lactate dehydrogenase may be used when the analyte of interest is lactate. Laccase may be used when the analyte of interest is oxygen or when oxygen is generated or consumed in response to a reaction of the analyte.

The sensing layer may also include a catalyst which is capable of catalyzing a reaction of the analyte. The catalyst may also, in some embodiments, act as an electron transfer agent. One example of a suitable catalyst is an enzyme which catalyzes a reaction of the analyte. For example, a catalyst, including a glucose oxidase, glucose dehydrogenase (e.g., pyrroloquinoline quinone (PQQ), dependent glucose dehydrogenase or oligosaccharide dehydrogenase, flavine adenine dinucleotide (FAD) dependent glucose dehydrogenase, nicotinamide adenine dinucleotide (NAD) dependent glucose dehydrogenase), may be used when the analyte of interest is glucose. A lactate oxidase or lactate dehydrogenase may be used when the analyte of interest is lactate. Laccase may be used when the analyte of interest is oxygen or when oxygen is generated or consumed in response to a reaction of the analyte.

In certain embodiments, a catalyst may be attached to a polymer, cross linking the catalyst with another electron transfer agent, which, as described above, may be polymeric. A second catalyst may also be used in certain embodiments. This second catalyst may be used to catalyze a reaction of a product compound resulting from the catalyzed reaction of the analyte. The second catalyst may operate with an electron transfer agent to electrolyze the product compound to generate a signal at the working electrode. Alternatively, a second catalyst may be provided in an interferent-eliminating layer to catalyze reactions that remove interferents.

In certain embodiments, the sensor includes the low surface tension polymer and works at a gentle oxidizing potential, e.g., a potential of about +40 mV vs. Ag/AgCl. This sensing layer uses, for example, an osmium (Os)-based mediator constructed for low potential operation and includes a low surface tension polymer. Accordingly, in certain embodiments the sensing element is a redox active component that includes (1) Osmium-based mediator molecules that include (bidente) ligands, and (2) glucose oxidase enzyme molecules. These two constituents are combined together with a low surface tension polymer.

A mass transport limiting layer (not shown), e.g., an analyte flux modulating layer, may be included with the sensor to act as a diffusion-limiting barrier to reduce the rate of mass transport of the analyte, for example, glucose or lactate, into the region around the working electrodes. The mass transport limiting layers are useful in limiting the flux of an analyte to a working electrode in an electrochemical sensor so that the sensor is linearly responsive over a large range of analyte concentrations and is easily calibrated. Mass transport limiting layers may include polymers and may be biocompatible. A mass transport limiting layer may provide many functions, e.g., biocompatibility and/or interferent-eliminating, etc.

In certain embodiments, a mass transport limiting layer is a membrane composed of crosslinked polymers containing heterocyclic nitrogen groups, such as polymers of polyvinylpyridine and polyvinylimidazole. In some embodiments, the low surface tension polymer is combined with the mass transport limiting layer or membrane. Embodiments also include membranes that are made of a polyurethane, or polyether urethane, or chemically related material, or membranes that are made of silicone, and the like.

A membrane may be formed by crosslinking in situ a polymer, modified with a zwitterionic moiety, a non-pyridine copolymer component, and optionally another moiety that is either hydrophilic or hydrophobic, and/or has other desirable properties, in an alcohol-buffer solution. In certain embodiments, the membrane formulation further includes a low surface tension polymer. The modified polymer may be made from a precursor polymer containing heterocyclic nitrogen groups. For example, a precursor polymer may be polyvinylpyridine or polyvinylimidazole. Optionally, hydrophilic or hydrophobic modifiers may be used to “fine-tune” the permeability of the resulting membrane to an analyte of interest. Optional hydrophilic modifiers, such as poly(ethylene glycol), hydroxyl or polyhydroxyl modifiers, may be used to enhance the biocompatibility of the polymer or the resulting membrane.

A membrane may be formed in situ by applying an alcohol-buffer solution of a crosslinker and a modified polymer over an enzyme-containing sensing layer and allowing the solution to cure for about one to two days or other appropriate time period. The crosslinker-polymer solution may be applied to the sensing layer by placing a droplet or droplets of the solution on the sensor, by dipping the sensor into the solution, or the like. Generally, the thickness of the membrane is controlled by the concentration of the solution, by the number of droplets of the solution applied, by the number of times the sensor is dipped in the solution, or by any combination of these factors. A membrane applied in this manner may have any combination of the following functions: (1) mass transport limitation, i.e., reduction of the flux of analyte that can reach the sensing layer, (2) biocompatibility enhancement, or (3) interferent reduction.

In certain embodiments, the sensing system detects hydrogen peroxide to infer glucose levels. For example, a hydrogen peroxide-detecting sensor may be constructed in which a sensing layer includes enzyme such as glucose oxides, glucose dehydrogenase, or the like, and is positioned proximate to the working electrode. The sensing layer may be covered by one or more layers, e.g., a membrane that is selectively permeable to glucose. Once the glucose passes through the membrane, it is oxidized by the enzyme and reduced glucose oxidase can then be oxidized by reacting with molecular oxygen to produce hydrogen peroxide.

Certain embodiments include a hydrogen peroxide-detecting sensor constructed from a sensing layer prepared by combining together, for example: (1) a redox mediator having a transition metal complex including an Os polypyridyl complexes with oxidation potentials of about +200 mV vs. SCE, (2) a low surface tension polymer, and (3) periodate oxidized horseradish peroxidase (HRP). Such a sensor functions in a reductive mode; the working electrode is controlled at a potential negative to that of the Os complex, resulting in mediated reduction of hydrogen peroxide through the HRP catalyst.

In another example, a potentiometric sensor can be constructed as follows. A glucose-sensing layer is constructed by combining together (1) a redox mediator having a transition metal complex including an Os polypyridyl complexes with oxidation potentials from about −200 mV to +200 mV vs. SCE, and (2) a low surface tension polymer, and (3) glucose oxidase. This sensor can then be used in a potentiometric mode, by exposing the sensor to a glucose containing solution, under conditions of zero current flow, and allowing the ratio of reduced/oxidized Os to reach an equilibrium value. The reduced/oxidized Os ratio varies in a reproducible way with the glucose concentration, and will cause the electrode's potential to vary in a similar way.

The substrate may be formed using a variety of non-conducting materials, including, for example, polymeric or plastic materials and ceramic materials. Suitable materials for a particular sensor may be determined, at least in part, based on the desired use of the sensor and properties of the materials.

In some embodiments, the substrate is flexible. For example, if the sensor is configured for implantation into a patient, then the sensor may be made flexible (although rigid sensors may also be used for implantable sensors) to reduce pain to the patient and damage to the tissue caused by the implantation of and/or the wearing of the sensor. A flexible substrate often increases the patient's comfort and allows a wider range of activities. Suitable materials for a flexible substrate include, for example, non-conducting plastic or polymeric materials and other non-conducting, flexible, deformable materials. Examples of useful plastic or polymeric materials include thermoplastics such as polycarbonates, polyesters (e.g., Mylar™ and polyethylene terephthalate (PET)), polyvinyl chloride (PVC), polyurethanes, polyethers, polyamides, polyimides, or copolymers of these thermoplastics, such as PETG (glycol-modified polyethylene terephthalate).

In other embodiments, the sensors are made using a relatively rigid substrate to, for example, provide structural support against bending or breaking. Examples of rigid materials that may be used as the substrate include poorly conducting ceramics, such as aluminum oxide and silicon dioxide. One advantage of an implantable sensor having a rigid substrate is that the sensor may have a sharp point and/or a sharp edge to aid in implantation of a sensor without an additional insertion device.

It will be appreciated that for many sensors and sensor applications, both rigid and flexible sensors will operate adequately. The flexibility of the sensor may also be controlled and varied along a continuum by changing, for example, the composition and/or thickness of the substrate.

In addition to considerations regarding flexibility, it is often desirable that implantable sensors should have a substrate which is physiologically harmless, for example, a substrate approved by a regulatory agency or private institution for in vivo use.

The sensor may include optional features to facilitate insertion of an implantable sensor. For example, the sensor may be pointed at the tip to ease insertion. In addition, the sensor may include a barb which assists in anchoring the sensor within the tissue of the patient during operation of the sensor. However, the barb is typically small enough so that little damage is caused to the subcutaneous tissue when the sensor is removed for replacement.

An implantable sensor may also, optionally, have an anticlotting agent disposed on a portion of the substrate which is implanted into a patient. This anticlotting agent may reduce or eliminate the clotting of blood or other body fluid around the sensor, particularly after insertion of the sensor. Blood clots may foul the sensor or irreproducibly reduce the amount of analyte which diffuses into the sensor. Examples of useful anticlotting agents include heparin and tissue plasminogen activator (TPA), as well as other known anticlotting agents.

The anticlotting agent may be applied to at least a portion of that part of the sensor that is to be implanted. The anticlotting agent may be applied, for example, by bath, spraying, brushing, or dipping. The anticlotting agent is allowed to dry on the sensor. The anticlotting agent may be immobilized on the surface of the sensor or it may be allowed to diffuse away from the sensor surface. Typically, the quantities of anticlotting agent disposed on the sensor are far below the amounts typically used for treatment of medical conditions involving blood clots and, therefore, have only a limited, localized effect.

Insertion Device

An insertion device can be used to subcutaneously insert the sensor into the patient. The insertion device is typically formed using structurally rigid materials, such as metal or rigid plastic. Exemplary materials include stainless steel and ABS (acrylonitrile-butadiene-styrene) plastic. In some embodiments, the insertion device is pointed and/or sharp at the tip to facilitate penetration of the skin of the patient. A sharp, thin insertion device may reduce pain felt by the patient upon insertion of the sensor. In other embodiments, the tip of the insertion device has other shapes, including a blunt or flat shape. These embodiments may be particularly useful when the insertion device does not penetrate the skin but rather serves as a structural support for the sensor as the sensor is pushed into the skin.

Sensor Control Unit

The sensor control unit can be integrated in the sensor, part or all of which is subcutaneously implanted or it can be configured to be placed on the skin of a patient. The sensor control unit is optionally formed in a shape that is comfortable to the patient and which may permit concealment, for example, under a patient's clothing. The thigh, leg, upper arm, shoulder, or abdomen are convenient parts of the patient's body for placement of the sensor control unit to maintain concealment. However, the sensor control unit may be positioned on other portions of the patient's body. One embodiment of the sensor control unit has a thin, oval shape to enhance concealment. However, other shapes and sizes may be used.

The particular profile, as well as the height, width, length, weight, and volume of the sensor control unit may vary and depends, at least in part, on the components and associated functions included in the sensor control unit. In general, the sensor control unit includes a housing typically formed as a single integral unit that rests on the skin of the patient. The housing typically contains most or all of the electronic components of the sensor control unit.

The housing of the sensor control unit may be formed using a variety of materials, including, for example, plastic and polymeric materials, particularly rigid thermoplastics and engineering thermoplastics. Suitable materials include, for example, polyvinyl chloride, polyethylene, polypropylene, polystyrene, ABS polymers, and copolymers thereof. The housing of the sensor control unit may be formed using a variety of techniques including, for example, injection molding, compression molding, casting, and other molding methods. Hollow or recessed regions may be formed in the housing of the sensor control unit. The electronic components of the sensor control unit and/or other items, including a battery or a speaker for an audible alarm, may be placed in the hollow or recessed areas.

The sensor control unit is typically attached to the skin of the patient, for example, by adhering the sensor control unit directly to the skin of the patient with an adhesive provided on at least a portion of the housing of the sensor control unit which contacts the skin or by suturing the sensor control unit to the skin through suture openings in the sensor control unit.

When positioned on the skin of a patient, the sensor and the electronic components within the sensor control unit are coupled via conductive contacts. The one or more working electrodes, counter electrode (or counter/reference electrode), optional reference electrode, and optional temperature probe are attached to individual conductive contacts. For example, the conductive contacts are provided on the interior of the sensor control unit. Other embodiments of the sensor control unit have the conductive contacts disposed on the exterior of the housing. The placement of the conductive contacts is such that they are in contact with the contact pads on the sensor when the sensor is properly positioned within the sensor control unit.

Sensor Control Unit Electronics

The sensor control unit also typically includes at least a portion of the electronic components that operate the sensor and the analyte monitoring device system. The electronic components of the sensor control unit typically include a power supply for operating the sensor control unit and the sensor, a sensor circuit for obtaining signals from and operating the sensor, a measurement circuit that converts sensor signals to a desired format, and a processing circuit that, at minimum, obtains signals from the sensor circuit and/or measurement circuit and provides the signals to an optional transmitter. In some embodiments, the processing circuit may also partially or completely evaluate the signals from the sensor and convey the resulting data to the optional transmitter and/or activate an optional alarm system if the analyte level exceeds a threshold. The processing circuit often includes digital logic circuitry.

The sensor control unit may optionally contain a transmitter for transmitting the sensor signals or processed data from the processing circuit to a receiver/display unit; a data storage unit for temporarily or permanently storing data from the processing circuit; a temperature probe circuit for receiving signals from and operating a temperature probe; a reference voltage generator for providing a reference voltage for comparison with sensor-generated signals; and/or a watchdog circuit that monitors the operation of the electronic components in the sensor control unit.

Moreover, the sensor control unit may also include digital and/or analog components utilizing semiconductor devices, including transistors. To operate these semiconductor devices, the sensor control unit may include other components including, for example, a bias control generator to correctly bias analog and digital semiconductor devices, an oscillator to provide a clock signal, and a digital logic and timing component to provide timing signals and logic operations for the digital components of the circuit.

As an example of the operation of these components, the sensor circuit and the optional temperature probe circuit provide raw signals from the sensor to the measurement circuit. The measurement circuit converts the raw signals to a desired format, using for example, a current-to-voltage converter, current-to-frequency converter, and/or a binary counter or other indicator that produces a signal proportional to the absolute value of the raw signal. This may be used, for example, to convert the raw signal to a format that can be used by digital logic circuits. The processing circuit may then, optionally, evaluate the data and provide commands to operate the electronics.

Calibration

Sensors may be configured to require no system calibration or no user calibration. For example, a sensor may be factory calibrated and need not require further calibrating. In certain embodiments, calibration may be required, but may be done without user intervention, i.e., may be automatic. In those embodiments in which calibration by the user is required, the calibration may be according to a predetermined schedule or may be dynamic, i.e., the time for which may be determined by the system on a real-time basis according to various factors, including, but not limited to, glucose concentration and/or temperature and/or rate of change of glucose, etc.

In addition to a transmitter, an optional receiver may be included in the sensor control unit. In some cases, the transmitter is a transceiver, operating as both a transmitter and a receiver. The receiver may be used to receive calibration data for the sensor. The calibration data may be used by the processing circuit to correct signals from the sensor. This calibration data may be transmitted by the receiver/display unit or from some other source such as a control unit in a doctor's office. In addition, the optional receiver may be used to receive a signal from the receiver/display units to direct the transmitter, for example, to change frequencies or frequency bands, to activate or deactivate the optional alarm system and/or to direct the transmitter to transmit at a higher rate.

Calibration data may be obtained in a variety of ways. For instance, the calibration data may simply be factory-determined calibration measurements which can be input into the sensor control unit using the receiver or may alternatively be stored in a calibration data storage unit within the sensor control unit itself (in which case a receiver may not be needed). The calibration data storage unit may be, for example, a readable or readable/writeable memory circuit.

Calibration may be accomplished using an in vitro test strip (or other reference), e.g., a small sample test strip such as a test strip that requires less than about 1 microliter of sample (for example FreeStyle® blood glucose monitoring test strips from Abbott Diabetes Care). For example, test strips that require less than about 1 nanoliter of sample may be used. In certain embodiments, a sensor may be calibrated using only one sample of body fluid per calibration event. For example, a user need only lance a body part one time to obtain sample for a calibration event (e.g., for a test strip), or may lance more than one time within a short period of time if an insufficient volume of sample is firstly obtained. Embodiments include obtaining and using multiple samples of body fluid for a given calibration event, where glucose values of each sample are substantially similar. Data obtained from a given calibration event may be used independently to calibrate or combined with data obtained from previous calibration events, e.g., averaged including weighted averaged, etc., to calibrate. In certain embodiments, a system need only be calibrated once by a user, where recalibration of the system is not required.

Alternative or additional calibration data may be provided based on tests performed by a doctor or some other professional or by the patient. For example, it is common for diabetic individuals to determine their own blood glucose concentration using commercially available testing kits. The results of this test is input into the sensor control unit either directly, if an appropriate input device (e.g., a keypad, an optical signal receiver, or a port for connection to a keypad or computer) is incorporated in the sensor control unit, or indirectly by inputting the calibration data into the receiver/display unit and transmitting the calibration data to the sensor control unit.

Other methods of independently determining analyte levels may also be used to obtain calibration data. This type of calibration data may supplant or supplement factory-determined calibration values.

In some embodiments of the invention, calibration data may be required at periodic intervals, for example, every eight hours, once a day, or once a week, to confirm that accurate analyte levels are being reported. Calibration may also be required each time a new sensor is implanted or if the sensor exceeds a threshold minimum or maximum value or if the rate of change in the sensor signal exceeds a threshold value. In some cases, it may be necessary to wait a period of time after the implantation of the sensor before calibrating to allow the sensor to achieve equilibrium. In some embodiments, the sensor is calibrated only after it has been inserted. In other embodiments, no calibration of the sensor is needed.

Analyte Monitoring Device

In some embodiments of the invention, the analyte monitoring device includes a sensor control unit and a sensor. In these embodiments, the processing circuit of the sensor control unit is able to determine a level of the analyte and activate an alarm system if the analyte level exceeds a threshold. The sensor control unit, in these embodiments, has an alarm system and may also include a display, such as an LCD or LED display.

A threshold value is exceeded if the datapoint has a value that is beyond the threshold value in a direction indicating a particular condition. For example, a datapoint which correlates to a glucose level of 200 mg/dL exceeds a threshold value for hyperglycemia of 180 mg/dL, because the datapoint indicates that the patient has entered a hyperglycemic state. As another example, a datapoint which correlates to a glucose level of 65 mg/dL exceeds a threshold value for hypoglycemia of 70 mg/dL because the datapoint indicates that the patient is hypoglycemic as defined by the threshold value. However, a datapoint which correlates to a glucose level of 75 mg/dL would not exceed the same threshold value for hypoglycemia because the datapoint does not indicate that particular condition as defined by the chosen threshold value.

An alarm may also be activated if the sensor readings indicate a value that is beyond a measurement range of the sensor. For glucose, the physiologically relevant measurement range is typically about 30 to 400 mg/dL, including about 40-300 mg/dL and about 50-250 mg/dL, of glucose in the interstitial fluid.

The alarm system may also, or alternatively, be activated when the rate of change or acceleration of the rate of change in analyte level increase or decrease reaches or exceeds a threshold rate or acceleration. For example, in the case of a subcutaneous glucose monitor, the alarm system might be activated if the rate of change in glucose concentration exceeds a threshold value which might indicate that a hyperglycemic or hypoglycemic condition is likely to occur.

A system may also include system alarms that notify a user of system information such as battery condition, calibration, sensor dislodgment, sensor malfunction, etc. Alarms may be, for example, auditory and/or visual. Other sensory-stimulating alarm systems may be used including alarm systems which heat, cool, vibrate, or produce a mild electrical shock when activated.

Drug Delivery System

The subject invention also includes sensors used in sensor-based drug delivery systems. The system may provide a drug to counteract the high or low level of the analyte in response to the signals from one or more sensors. Alternatively, the system may monitor the drug concentration to ensure that the drug remains within a desired therapeutic range. The drug delivery system may include one or more (e.g., two or more) sensors, a processing unit such as a transmitter, a receiver/display unit, and a drug administration system. In some cases, some or all components may be integrated in a single unit. A sensor-based drug delivery system may use data from the one or more sensors to provide necessary input for a control algorithm/mechanism to adjust the administration of drugs, e.g., automatically or semi-automatically. As an example, a glucose sensor may be used to control and adjust the administration of insulin from an external or implanted insulin pump.

Each of the various references, presentations, publications, provisional and/or non-provisional U.S. Patent Applications, U.S. patents, non-U.S. Patent Applications, and/or non-U.S. patents that have been identified herein, is incorporated herein in its entirety by this reference.

Other aspects, advantages, and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains. Various modifications, processes, as well as numerous structures to which the embodiments of the invention may be applicable will be readily apparent to those of skill in the art to which the invention is directed upon review of the specification. Various aspects and features of the invention may have been explained or described in relation to understandings, beliefs, theories, underlying assumptions, and/or working or prophetic examples, although it will be understood that the invention is not bound to any particular understanding, belief, theory, underlying assumption, and/or working or prophetic example. Although various aspects and features of the invention may have been described largely with respect to applications, or more specifically, medical applications, involving diabetic humans, it will be understood that such aspects and features also relate to any of a variety of applications involving non-diabetic humans and any and all other animals. Further, although various aspects and features of the invention may have been described largely with respect to applications involving partially implanted sensors, such as transcutaneous or subcutaneous sensors, it will be understood that such aspects and features also relate to any of a variety of sensors that are suitable for use in connection with the body of an animal or a human, such as those suitable for use as fully implanted in the body of an animal or a human. Finally, although the various aspects and features of the invention have been described with respect to various embodiments and specific examples herein, all of which may be made or carried out conventionally, it will be understood that the invention is entitled to protection within the full scope of the appended claims.

The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to make and use the embodiments of the invention, and are not intended to limit the scope of what the inventors regard as their invention nor are they intended to represent that the experiments below are all or the only experiments performed. Efforts have been made to ensure accuracy with respect to numbers used (e.g. amounts, temperature, etc.) but some experimental errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, molecular weight is weight average molecular weight, temperature is in degrees Centigrade, and pressure is at or near atmospheric.

EXAMPLE 1 Sensors Having Sensing Layers Incorporating a Low Surface Tension Polymer

FIG. 6A illustrates problems that may be associated with sensor membrane fabrication. The figure shows the so-called “coffee ring” effect which results in poor uniformity and/or distribution of one or more components of the sensing layer of a sensor (e.g., an enzyme-containing sensing layer of such devices). In order to address these issues a low surface tension polymer was incorporated into the sensing layer of the sensor.

In a comparison between a sensing layer formulation deposited on a gold substrate in which the sensing layer formulation was not supplemented with a low surface tension polymer and a sensing layer formulation deposited on a gold substrate in which a sensing layer formulation that was treated with a low surface tension polymer, the low surface tension polymer-treated sensing layer is smoother, more homogenous and uniform in distribution (see FIG. 6B) as compared to the sensing layer lacking the low surface tension polymer (see FIG. 6A). Supplementing the sensing layer formulation with a low surface tension polymer results in an improved sensing layer coating that eliminates a lack of uniformed distribution and the “coffee ring” effect as shown in FIG. 6B.

When FIG. 6A is juxtaposed against FIG. 6B, they provide a comparison between a sensing layer formulation deposited on a gold substrate in which the sensing layer formulation was not supplemented with a low surface tension polymer (FIG. 6A), versus a sensing layer formulation deposited on a gold substrate with a sensing layer that was supplemented with the low surface tension polymer dimethyl, methyl (polyethylene oxide acetate-capped) siloxane (“DC 57”), (FIG. 6B). In addition, as evidenced by FIGS. 7 and 8, treatment with different low surface tension polymers, such as DC28 (FIG. 7) and Surfynol® 2502 (FIG. 8) also resulted in a sensing layer that is smooth, homogenous and uniform in distribution.

Table 1 summarizes data relating to contact angle and percent change in contact angle as compared to a control sensing layer formulation for sensing layers supplemented with the low surface tension polymer DC 57 at either 1% or 2% of total volume. As summarized below, inclusion of a low surface tension polymer at 1% of total volume decreases the contact angle by 32.7% and inclusion of a low surface tension polymer at 2% of total volume decreases the contact angle by 31.1%.

TABLE 1 Sample Contact Angle % Change Control 61° — 1% DC 57 41° −32.7 2% DC 57 42° −31.1

Table 2 summarizes data for three groups of sensors, including coefficient of variation between different sensors (% CV), time for response to analyte levels (time (response/min), sensitivity (nA/mM), and decay. The first group of sensors is control sensors having sensing layers that lack a low surface tension polymer. The second and third groups had sensing layers that were supplemented with either 1% DC 57 (group 2) or 3% DC 57 (group 3). Sensors from each group were then tested after manufacture (0 days) or after storing for 11 days at 4° C. or 11 days at 56° C. to determine stability of the sensors.

TABLE 2 Group 1 - Control Group 2 - 1% DC 57 Group 3 - 3% DC 57 11 Days 11 Days 11 Days 11 Days 11 Days 11 Days 0 Days @4° C. @56° C. 0 Days @4° C. @56° C. 0 Days @4° C. @56° C. Number of 8 8 8 8 8 8 7 8 8 sensors nA/mM 0.578 0.562 0.452 0.471 0.502 0.408 0.591 0.66 0.502 % CV 11.5 10.2 6.3 10.6 6.2 6.6 6.7 12.3 5.3 nA/mM * mm² 8.25 8.02 6.46 6.73 7.17 6.01 8.44 9.43 7.17 Time 2:48 3:19 4:01 5:10 4:47 5:44 4:42 4:52 5:16 (response/min) Decay/%/hr −0.04 0.06 −0.03 0.12 0.05 0.03 0.19 0.53 0.38 Decay - −0.1 −0.06 −0.07 −0.06 −0.12 −0.12 0.01 0.11 0.09 no Hump Slope −19.6% −18.7% −23.9% Changed

FIG. 10 shows the results of sensitivity comparison (nA/mm²) at varying concentrations of glucose (mM) between control sensors (diamonds), sensors with the sensing layers supplemented with 1% of the low surface tension polymer DC 57 (squares) and sensors with the sensing layers supplemented with 3% of the low surface tension polymer DC 57 (triangles). As demonstrated in FIG. 10, the low surface tension polymer supplemented sensing layers sensors had a similar sensitivity to those lacking the low surface tension polymer.

EXAMPLE 2 Sensors Having Membrane Incorporating a Low Surface Tension Polymer

FIG. 9 illustrates problems that may be associated with sensor membrane fabrication, such as a polymeric glucose-limiting membrane formulation that includes a heterocyclic nitrogen containing polymer (see for example U.S. Pat. No. 6,932,894). The figure shows the so-called “orange peel” effect of the membrane coating, which results in poor uniformity of the membrane on the substrate, such as a carbon substrate. In order to address this issue a low surface tension polymer was incorporated into the polymer membrane of the sensor.

In a comparison between a control sensor in which the membrane formulation was not supplemented with a low surface tension polymer and a sensor in which a membrane formulation that was treated with a low surface tension polymer, the low surface tension polymer-treated sensor membrane is smoother, and more homogenous (see FIG. 9B) as compared to the sensor membrane lacking the low surface tension polymer (see FIG. 9A). Supplementing the membrane formulation with a low surface tension polymer results in an improved membrane coating that eliminates wrinkling and levels the “orange peel” effect as shown in FIG. 9B. When FIG. 9A is juxtaposed against FIG. 9B, they provide a comparison between a control sensor in which the membrane formulation was not supplemented with a low surface tension polymer 10 days post cure at 56° C. (FIG. 9A), versus a sensor with a membrane coating that was supplemented with the low surface tension polymer DC 57 at 0.05% of total volume (FIG. 9B). After 16 days at 56° C., the DC 57-treated sensor membrane appears smooth and homogenous.

In conclusion, the experiments above show that the addition of one or more low surface tension polymers to an analyte sensor formulation, such as a sensing layer or a polymeric glucose-limiting membrane, promotes the uniformity and/or distribution of one or more components of the membrane formulation and elimination of the “coffee ring” effect of settling of sensing layer components at the perimeter of the formulation on the sensor surface. Furthermore, the addition of one or more low surface tension polymers to an analyte sensor membrane formulation also promotes the uniformity of the membrane, eliminating the “orange peel” effect and wrinkles, thus improving the manufacturing yield. 

That which is claimed is:
 1. An analyte sensor comprising: a working electrode comprising a sensing layer disposed proximate thereto, wherein the sensing layer comprises a hydrophilic polymer; a membrane disposed over the sensing layer comprising a low surface tension polymer, wherein the low surface tension polymer comprises 1% or less by weight of the total formulation of the membrane wherein the low surface tension polymer is polydimethylsiloxane (H₃C)₃Si[Si(CH₃)₂O]_(n)Si(CH₃)₃, polyether modified polydimethylsiloxane [CH₂O—], [Si(CH₃)₂O]_(n)—[CH₂O—]_(n), poly((eo)(po))butyl terminated PDMS CH₃—{[CH₂CH₂O]_(n)—[CH₂O—]_(n)}_(n)[Si(CH₃)₂O]_(n)—{[CH₂O—]_(n)[CH₂CH₂O]_(n)}_(n)—CH₃; or dimethyl, methyl (polyethylene oxide acetate-capped) siloxane (DC 57), wherein n is an integer in a range of from about 1 to 500; and a counter electrode.
 2. The analyte sensor of claim 1, wherein at least a portion of the electrochemical analyte sensor is adapted to be subcutaneously positioned in a subject.
 3. The analyte sensor of claim 1, wherein the low surface tension polymer comprises a substituted or unsubstituted polyacrylate polymer or a substituted or unsubstituted polysiloxane polymer.
 4. The analyte sensor of claim 1, wherein the low surface tension polymer comprises polyether-modified polydimethylsiloxane.
 5. The analyte sensor of claim 1, wherein the sensing layer comprises a glucose-responsive enzyme.
 6. The analyte sensor of claim 1, wherein the sensing layer comprises a redox mediator.
 7. The analyte sensor of claim 6, wherein the redox mediator comprises a ruthenium-containing complex or an osmium-containing complex.
 8. The analyte sensor of claim 6, wherein the sensor is a glucose sensor.
 9. The analyte sensor of claim 6, wherein the sensor is an in vivo sensor.
 10. The analyte sensor of claim 6, wherein the sensor is an in vitro sensor.
 11. A method for monitoring a level of an analyte in a subject, the method comprising: positioning at least a portion of an analyte sensor into skin of a subject, wherein the sensor comprises: a working electrode comprising a sensing layer proximate thereto, wherein the sensing layer comprises a hydrophilic polymer; a membrane disposed over the sensing layer comprising a low surface tension polymer, wherein the low surface tension polymer comprises 1% or less by weight of the total formulation of the membrane wherein the low surface tension polymer is polydimethylsiloxane (H₃C)₃Si[Si(CH₃)₂O]_(n)Si(CH₃)₃, polyether modified polydimethylsiloxane [CH₂O—], [Si(CH₃)₂O]_(n)—[CH₂O—]_(n), poly((eo)(po))butyl terminated PDMS CH₃—{[CH₂CH₂O]_(n)—[CH₂O—]_(n)}_(n)[Si(CH₃)₂O]_(n)—{[CH₂O—]_(n)[CH₂CH₂O]_(n)}_(n)—CH₃; or dimethyl, methyl (polyethylene oxide acetate-capped) siloxane (DC 57), wherein n is an integer in a range of from about 1 to 500; a counter electrode; and determining a level of an analyte over a period of time from signals generated by the sensor; wherein the determining over a period of time provides for monitoring the level of the analyte in the subject.
 12. The method of claim 11, wherein the low surface tension polymer is a substituted or unsubstituted polyacrylate polymer or a substituted or unsubstituted polysiloxane polymer.
 13. The method of claim 11, wherein the sensing layer comprises a glucose-responsive enzyme.
 14. The method of claim 11, wherein the sensing layer comprises a redox mediator.
 15. The method of claim 14, wherein the redox mediator comprises a ruthenium-containing complex or an osmium-containing complex. 